Solid State Fluorination on the Minute Scale: Synthesis of WO₃₋ₓFx with Photocatalytic Activity

Solid state reactions are notoriously slow, because the rate‐limiting step is diffusion of atoms or ions through reactant, intermediate, and product crystalline phases. This requires days or even weeks of high temperature treatment, consuming large amounts of energy. Metal oxides are particularly difficult to react, because they have high melting points. The study reports a high‐speed solid state fluorination of WO₃ with Teflon to the oxyfluorides WO₃₋ₓFx on a minute (<10 min) scale by spark plasma sintering, a technique that is used typically for a high‐speed consolidation of powders. Automated electron diffraction analysis reveals an orthorhombic ReO₃‐type structure of WO₃₋ₓFx with F atom disorder as demonstrated by ¹⁹F magic angle spinning nuclear magnetic resonance spectroscopy. The potential of this new approach is demonstrated by the following results. i) Mixed‐ valent tungsten oxide fluorides WO₃₋ₓFx with high F content (0 < x < 0.65) are obtained as metastable products in copious amounts within minutes. ii) The spark plasma sintering technique yields WO₃₋ₓFx nanoparticles with high photocatalytic activity, whereas the corresponding bulk phases obtained by conventional solid‐state (ampoule) reactions have no photocatalytic activity. iii) The catalytic activity is caused by the microstructure originating from the processing by spark plasma sintering

Influence of Compensating Defect Formation on the Doping Efficiency and Thermoelectric Properties of Cu_(2-y)Se_(1–x)Br_x

The superionic conductor Cu_(2−δ)Se has been shown to be a promising thermoelectric at higher temperatures because of very low lattice thermal conductivities, attributed to the liquid-like mobility of copper ions in the superionic phase. In this work, we present the potential of copper selenide to achieve a high figure of merit at room temperature, if the intrinsically high hole carrier concentration can be reduced. Using bromine as a dopant, we show that reducing the charge carrier concentration in Cu_(2−δ)Se is in fact possible. Furthermore, we provide profound insight into the complex defect chemistry of bromine doped Cu_(2−δ)Se via various analytical methods and investigate the consequential influences on the thermoelectric transport properties. Here, we show, for the first time, the effect of copper vacancy formation as compensating defects when moving the Fermi level closer to the valence band edge. These compensating defects provide an explanation for the often seen doping inefficiencies in thermoelectrics via defect chemistry and guide further progress in the development of new thermoelectric materials

Diffusion-driven formation of MoS2 nanotube bundles containing MoS2 nanopods

MoS2 nanotube bundles along with embedded nested fullerenes were formed in a gas phase reaction of molybdenum carbonyl and H2S gas with the assistance of I2. The amorphous Mo-S-I intermediates obtained through quenching a modified MOCVD reaction in a large temperature gradient were annealed at elevated temperature in an inert atmosphere. Under the influence of the iodine the amorphous precursor formed a surface film with an enhanced mobility of the molybdenum and sulfur components. Point defects within the MoS2 layers combined with the enhanced surface diffusion lead to a scrolling of the inherently instable MoS2 lamellae

Solution synthesis of a new thermoelectric Zn(1+x)Sb nanophase and its structure determination using automated electron diffraction tomography.

Engineering materials with specific physical properties have recently focused on the effect of nanoscopic inhomogeneities at the 10 nm scale. Such features are expected to scatter medium- and long-wavelength phonons thereby lowering the thermal conductivity of the system. Low thermal conductivity is a prerequisite for effective thermoelectric materials, and the challenge is to limit the transport of heat by phonons, without simultaneously decreasing charge transport. A solution-phase technique was devised for synthesis of thermoelectric "Zn(4)Sb(3)" nanocrystals as a precursor for phase segregation into ZnSb and a new Zn-Sb intermetallic phase, Zn(1+delta)Sb, in a peritectoid reaction. Our approach uses activated metal nanoparticles as precursors for the synthesis of this intermetallic compound. The small particle size of the reactants ensures minimum diffusion paths, low activation barriers, and low reaction temperatures, thereby eliminating solid-solid diffusion as the rate-limiting step in conventional bulk-scale solid-state synthesis. Both phases were identified and structurally characterized by automated electron diffraction tomography combined with precession electron diffraction. An ab initio structure solution based on electron diffraction data revealed two different phases. The new pseudo-hexagonal phase, Zn(1+delta)Sb, was identified and classified within the structural diversity of the Zn-Sb phase diagram

Snapshots of the Formation of NaTi3O6(OH)·2H2O Nanowires: A Time-Resolved XRD/HRTEM Study

Layered titanates are important intermediates during the formation of TiO2-related nanostructures in hot concentrated base solution. Microwave-assisted hydrothermal techniques allow a time-resolved ex-situ analysis of the reaction in one-minute intervals by rapid heating and quenching followed by separation and structure analysis of the intermediates. By a combination of powder X-ray diffraction, high resolution electron microscopy (HRTEM), and selected area electron diffraction (SAED) the individual stages of the reaction could be identified. Sodium titanate nanosheets are formed within several minutes by digesting the crystalline TiO2-P25 precursor in NaOH. These nanosheets with a low sodium content form flake-like aggregates. Subsequently, these nanosheet intermediates form nanoscrolls and/or nanotubes, which transform in the next step to NaTi3O6(OH) ·2H2O nanowires by reaction with NaOH via an oriented attachment of nanotubes from solution. The NaTi3O6(OH) ·2H2O nanowires grow at the expense of the nanotubes and form an ordered structure via dissolution-recrystallization equilibria between solid NaTi3O6(OH)·2H2O and soluble titanate species. Copyright © 2013 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim

Blue light mediated C-H arylation of heteroarenes using TiO2 as an immobilized photocatalyst in a continuous-flow microreactor

Titanium dioxide was applied as an immobilized photocatalyst in a microstructured falling film reactor for the continuous-flow C–H arylation of heteroarenes with aryldiazonium salts as the starting material. Detailed investigations of the catalyst and a successful long-term run proved its excellent usability for this process. Very good yields up to 99% were achieved with broad substrate scope and were compared with batch synthesis. The transfer to the continuous-flow mode revealed an impressive boost in reactor performance solely resulting from the improved irradiation and contact of the catalyst, substrate and light

Thermoelectric Transport in Cu_7PSe_6 with High Copper Ionic Mobility

Building on the good thermoelectric performances of binary superionic compounds like Cu_2Se, Ag_2Se and Cu_2S, a better and more detailed understanding of phonon-liquid electron-crystal (PLEC) thermoelectric materials is desirable. In this work we present the thermoelectric transport properties of the compound Cu_7PSe_6 as the first representative of the class of argyrodite-type ion conducting thermoelectrics. With a huge variety of possible compositions and high ionic conductivity even at room temperature, the argyrodites represent a very good model system to study structure–property relationships for PLEC thermoelectric materials. We particularly highlight the extraordinary low thermal conductivity of Cu_7PSe_6 below the glass limit, which can be associated with the molten copper sublattice leading to a softening of phonon modes